The present invention is directed toward the field of solid state temperature sensors. In particular, a high-sensitivity diode temperature sensor circuit is disclosed that preferably comprises a reverse-biased diode, such as a Schottky diode, coupled to an adjustable constant current source for biasing the diode into a reverse operating region. The constant current source biases the diode at a particular reverse leakage current that corresponds to a temperature window over which the reverse voltage across the diode exhibits a linear response of several hundred mV/C. The adjustable reverse leakage current sets the beginning of the temperature window over which the diode""s reverse voltage will respond. This large change in diode voltage (from 100-500 mV/C) over a relatively small temperature window (from 5-20 xc2x0 C.) can be used as a signal to switch power to an attached electrical load.
In addition to disclosing the temperature sensor, the present application describes a control circuit that incorporates, as one element, the high sensitivity diode temperature sensor. The control circuit includes an innovative feedback mechanism that, in combination with the temperature sensor circuit, enables the controller to switch a load on and off at two adjustable set points using the single temperature sensor.
Presently known solid state temperature sensor circuits include: (1) integrated circuit temperature sensors; (2) forward-biased diode temperature sensors; (3) NTC/PTC thermistor circuits; and (4) complex reverse-biased diode temperature sensors that lack sensitivity and adjustability. Each of these presently known methods of measuring temperature suffer from several disadvantages that make them commercially or technically undesirable.
Integrated circuit (xe2x80x9cICxe2x80x9d) temperature sensors typically measure temperature using a pair of back-to back forward-biased diode junctions. The difference in the biasing current level between the two diodes indicates the sensed temperature. Examples of these types of IC sensors include the TMP12 from Analog Devices and the LM 34/35, available from National Semiconductor. These types of IC sensors suffer from several disadvantages. First, they are complex circuits that require external biasing and range-setting components, and thus consume valuable real estate on a printed circuit board (xe2x80x9cPCBxe2x80x9d). Second, they exhibit a relatively low temperature sensitivity (measured as the voltage output per degree of temperature change) in the range of only 5 to 20 mV/C. And third, they are expensive in comparison to discrete component circuits.
It is also known to use a discrete forward-biased diode as a temperature sensor. This type of sensor is disadvantageous, however, because a forward-biased diode exhibits a nonlinear change in output voltage with respect to temperature, and because the temperature sensitivity of such a forward-biased junction is very low, on the order of only xe2x88x922 mV/C. In addition, the forward voltage drop from diode to diode in a given lot is generally inconsistent, which means that the biasing circuitry needed to operate such a diode must be customized for each sensor, if the circuits are to operate over the same temperature ranges. Furthermore, sensors that employ forward-biased diodes are generally not easily adjusted to switch at a different temperature point.
Another known solid state temperature sensor is the thermistor, either NTC or PTC. The thermistor circuit is relatively inexpensive and uncomplicated, however, it lacks adjustability. In addition, thermistor circuits that are biased to provide a narrow temperature xe2x80x9cwindowxe2x80x9d over which switching takes place generally require expensive high-gain amplification circuits.
Several prior art patents describe attempts to develop a high-sensitivity, adjustable solid state temperature sensor using a reverse-biased diode. These patents include U.S. Pat. No. 5,070,322 to Fujihira (xe2x80x9cFujihiraxe2x80x9d), U.S. Pat. No. 3,719,797 to Andrews (xe2x80x9cAndrewsxe2x80x9d), and U.S. Pat. No. 3,420,104 to Troemel (xe2x80x9cTroemelxe2x80x9d).
Fujihira describes an overheating detection circuit including a reverse biased-diode coupled to a series of current amplification states that amplify the reverse leakage current (IL) and provide this current to a MOSFET that converts the amplified current (IF) to a voltage. Fujihira does not include an adjustable constant current source that can be used to program the temperature setpoint at which the device switches, and, in addition, requires a costly and complex series of emitter-follower transistors for amplifying the leakage current of the diode sensor.
Andrews describes a sensor employing a pair of series connected reverse-biased Schottky diodes having dissimilar barrier heights. This circuit is not easily adjustable, does not employ a constant current source, and does not operate linearly over a particular temperature range. In addition, it requires the precise selection of two diodes having particular barrier heights.
Troemel describes a temperature sensor using a zener diode biased into its reverse-breakdown region. This circuit has a relatively poor sensitivity, does not employ an adjustable constant current source, and its temperature switch point is not easily changed.
Therefore, there remains a need in the art of solid state temperature sensors for an inexpensive, adjustable high sensitivity temperature measurement device that exhibits a linear output response over a narrow temperature window that can be used as a signal to switch a motor, heater, lamp or other component that could be damaged by operating outside the temperature window.
Another leading concern with temperature control in heater-based appliances such as hot water dispensers, coffee makers, slow cookers and hot water controllers, is the need for dry start protection. For example, if a hot water dispenser is turned on while empty, significant damage can be done to the vessel, heating element, and other surrounding devices. Thus, it is desirable to prevent the heating element from energizing under certain conditions. It is also desirable to provide a latching mechanism which requires a manual reset before heating can continue. This feature would allow additional protection. Relay drivers are also often used in conjunction with temperature control circuitry. It is therefore desirable to provide temperature control which can be switched to a relay.
The present invention overcomes the problems noted above and satisfies the needs in this field for a solid state temperature sensor that is inexpensive, adjustable, and exhibits a linear response and high temperature sensitivity over a programmable window of operation. The preferred sensor is a reverse-biased diode, and in particular a reverse-biased Schottky diode. Coupled to the reverse diode is a constant current source that includes an adjustable component for setting the position of the switching window over which the diode exhibits a linear change in voltage with respect to temperature.
Certain types of mechanical devices, such as motors, heaters, lamps, compressors, etc., can be damaged if they are operated in an ambient environment that is either too hot or too cold. The present invention provides an adjustable temperature sensor that, in combination with a logic circuit and power switch, can be used to protect one of these mechanical devices by supplying a voltage level to the logic circuit that causes the power switch to remove power from the protected device. This sensor is highly immune to noise, and is therefore well suited for use in an appliance or other noisy environment, due to the fact that it exhibits a high temperature sensitivity of several hundred millivolts per degree Celsius over a narrow temperature window. The sensor can easily be used in a variety of applications and ambient temperature environments due to its included adjustable current source that is used to program the sensor to transition from a high reverse voltage to a low reverse voltage in a linear fashion over the temperature window.
A preferred application of the high sensitivity diode temperature sensor is a controller for controlling the application and removal of power from a load in a refrigeration, heat pump or air conditioning application, such as a defrost heater for a freezer, although, alternatively, the sensor can be used with a wide variety of appliances and other systems that need to switch power to a particular mechanical or electrical device when the ambient temperature of the system exceeds or falls below a particular level. As used in the preferred control circuit, the present invention includes the high-sensitivity diode temperature sensor, a feedback adjustment circuit, driver transistor, and a relay. The combination of the diode temperature sensor circuitry and the feedback adjustment circuit enables the preferred controller to switch at two adjustable temperature setpoints using only a single diode temperature sensor.
The present invention provides many advantages over presently known solid state temperature sensors, including: (1) the output of the sensor exhibits a linear voltage response over a relatively small temperature window, which can be used as a signal to switch power to a load; (2) the sensor is low cost and utilizes discrete components; (3) the sensor output is adjustable by altering the reverse leakage current provided by the constant current source; (4) the sensor provides a relatively high temperature sensitivity in the range of 100-500 mV/C over the linear temperature window; (5) when used as a temperature measuring device (and not a switch), the sensor provides an accuracy of about +/xe2x88x920.1xc2x0 C.; and (6) the sensor is small in size, inexpensive to build and operate, and exhibits consistent operating characteristics from sensor to sensor.
In another preferred application, the diode temperature sensor of the present invention can be used in multiple sensor configurations for dry start protection, with multiple optical isolation devices for separate load control, with latching and manual reset circuitry, and with improved relay control systems. For example, the present invention provides a temperature control circuit for removing power from a load at a first temperature. The temperature control circuit has a first optical isolation device controlling operation of a power source, wherein the power source provides power to the load. A first diode temperature sensor is biased to provide a switching signal at the first temperature. The temperature control circuit further includes a first switching mechanism disposed between the first diode temperature sensor and the first optical isolation device. The first switching mechanism disengages operation of the first optical isolation device in response to the switching signal. This configuration allows control of a low-cost power supply with a triac. Furthermore, a high limit cycling circuit disengages operation of the first optical isolation device at a second temperature. This allows dry start protection through multiple sensing diodes with unique trip-point temperatures.
The present invention further provides for a latching circuit for maintaining a temperature control circuit in a latched state until a manual reset occurs. The latching circuit includes a bipolar transistor shunted across an optical isolation device, a silicon controlled rectifier connected in series with the bipolar transistor, and a manual reset switch connected in series with the optical isolation device. As an additional aspect of the invention, a method is provided for removing power from a load at a first temperature. The method includes the steps of controlling operation of a power source with a first optical isolation device, and biasing a first diode temperature sensor to provide a switching signal at the first temperature. Operation of the first optical isolation device is disengaged in response to the switching signal. The method further provides for disengaging operation of the power source at a second temperature.
These are just a few of the many advantages of the present invention, as described in more detail below. As will be appreciated, the invention is capable of other and different embodiments, and its several details are capable of modifications in various respects, all without departing from the spirit of the invention. Accordingly, the drawings and description of the preferred embodiments set forth below are to be regarded as illustrative in nature and not restrictive.